This invention relates generally to fluid line coupling devices, and more particularly, to fluid line coupling devices used to connect and disconnect relacement modules in hydraulic and other fluid systems. To facilitate maintenance of a fluid system, it is often desirable to design the system to include one or more replaceable modules, which can be conveniently removed and replaced without having to drain fluid from the system, and without having to refill the system and flush it to remove trapped air.
A fluid system designed to include replaceable modules requires a special type of fluid coupling device having two essential properties. First, little or no air must be introduced into the fluid system during connection of the coupling, and second, there must be little or no spillage of fluid during connection or disconnection. Couplings of this general type have been available for some years, as disclosed, for example, in U.S. Pat. Nos. 3,073,342, entitled "Airless Coupling," and 3,334,659, entitled "Flow Stream Immersed Supporting Structure", both invented by Vincent G. Magorien.
Although fluid couplings providing negligible fluid loss are desirable in many applications, there is an inherent disadvantage to the use of these couplings. Fluid modules that can be connected and disconnected with practically no volume change constitute a relatively "stiff" system from a fluid mechanics standpoint. Some form of pressure relief has to be provided to compensate for fluid volume changes caused by increases in temperature, for example. The relief may be provided by a reservoir somewhere in the system, or by a thermally actuated pressure relief valve. Since a reservoir is more costly to provide than a relief valve, most systems include a reservoir only in the principal module of the system, and include relief valves in the removable modules. However, when a module with a relief valve is coupled to a principal module with an expansion reservoir, the relief valve becomes redundant. Another drawback is that a relief valve may occasionally open in the coupled condition, and lead operators of the system to conclude that there is an unwanted leak in the system.
Accordingly, it will be apparent that there has been a significant need for a coupling component that functions as a relief valve when in the disconnected condition, but functions as a conventional coupling component when in the connected condition. Ideally, such a coupling component should employ substantially the same parts as a conventional coupling component without the relief valve feature, so that manufacturing costs would not be significantly affected. The present invention is directed to these ends.